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Investments (Net Realized Investments Gains) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Investments [Line Items] |  |  |
Net realized investment gains (losses) | $ 81,184 us-gaap_RealizedInvestmentGainsLosses | $ (11,422)us-gaap_RealizedInvestmentGainsLosses | $ 66,380 us-gaap_RealizedInvestmentGainsLosses
Swap [Member] |  |  |
Investments [Line Items] |  |  |
Net realized investment gains (losses) | (2,969)us-gaap_RealizedInvestmentGainsLosses / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember | 2,176 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember | 0 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember
Fixed Maturity Securities [Member] |  |  |
Investments [Line Items] |  |  |
Net realized investment gains (losses) | 74,489 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember | (95,225)us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember | 47,707 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember
Equity Securities [Member] |  |  |
Investments [Line Items] |  |  |
Net realized investment gains (losses) | 8,715 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember | 80,910 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember | 16,679 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember
Short-Term Investments [Member] |  |  |
Investments [Line Items] |  |  |
Net realized investment gains (losses) | (2,470)us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember | (1,059)us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember | (686)us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember
Option [Member] |  |  |
Investments [Line Items] |  |  |
Net realized investment gains (losses) | $ 3,419 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = mcy_OptionAndShortSaleTransactionsMember | $ 1,776 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = mcy_OptionAndShortSaleTransactionsMember | $ 2,680 us-gaap_RealizedInvestmentGainsLosses / us-gaap_DebtSecurityAxis = mcy_OptionAndShortSaleTransactionsMember

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- Definition The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 325 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6485959&loc=d3e22529-158502 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 3 -Subparagraph a -Article 7 + Details
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